This application claims the priority of German patent document 199 55 609.1, filed 19 Nov. 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a system for infrared (IR) camouflaging of land targets, especially military objects, such as land craft, against thermal-image apparatuses and infrared seeker heads.
The objective of thermal camouflaging is to adapt the thermal radiation emitted by an object which is to be camouflaged, to the level of the respective thermal background, for example by influencing the temperature of the observable surfaces using constructional measures, such as thermal insulation, insulation and rear ventilation. These measures can achieve improvements in the area of an active signature (that is, for internal heat sources, such as engines, transmissions or energy units); however, they do not attain a satisfactory solution with respect to solar heating (passive signature), because the heating behavior of military objects as a rule deviates considerably from that of a natural background. Suggested solutions for compensating these deviations by active afterheating and cooling, such as described, for example, in German Patent Document DE 32 17 977 A1, are not very practical, mainly because of the high energy consumption.
Other known solutions have the goal of achieving a signature reduction by changing the emission behavior of the surface rather than by influencing the actual surface temperature. It is known that the heat emission of a body is determined not only by its temperature but also by the thermal emissivity of its surface. The use of low-emitting surface layers for infrared camouflage is known and described, for example, in German Patent Document DE 30 43 381 A1 and European Patent Document EP 0 123 660 A1.
One problem encountered with this type of low-emitting camouflaging devices is that in principle the IR reflectivity p increases with a reduction of the thermal emissivity xcex5 according to the formula xcfx81=1xe2x88x92xcex5, so that reflection of the environmental radiation increases. This environmental (reflected) radiation is superimposed on intrinsic emissions, so that the heat radiation (and thus the observable radiation temperature during the reduction of the thermal emissivity) is increasingly also dependent on the temperatures of the reflected ambient surfaces (ground temperature, celestial temperature). In particular, reflections from celestial areas close to the zenith have been found to be critical because, depending on the cloudiness, the radiation temperatures are considerably different and can significantly influence the signature. A known effect in the case of low-emitting camouflaging devices is the observation of cold spots (that is, surface areas with a radiation temperature which is too low with respect to the background, due to the reflection of cold celestial areas).
In order to take this condition into account, European Patent Document EP 0 250 742 A1 describes a system which controls the thermal emissivity so that the heat radiation of an object can be adjusted within wide limits as desired, by controlling the heat reflection and emission fractions by virtue of a very low energy consumption. This permits a considerable contrast reduction of the thermal radiation with respect to the background. However, the high expenditures for implementing corresponding systems and the necessity of providing additional measuring and regulating devices are disadvantageous.
When low-emitting infrared camouflage devices are used, the geometrical features of the object to be camouflaged must be taken into account. For this purpose, a distinction must be made between:
surface areas inclined toward the ground;
horizontal surface areas or those which are inclined toward the sky; and
surface areas which are vertical or incline slightly (up to approximately 25xc2x0) toward the sky.
These surface areas require different embodiments of the camouflaging devices. For surfaces which slope predominantly toward the ground, low-emitting camouflaging devices can be used with a firmly adjusted emissivity which is as low as possible, because the ground temperatures situated in front of the object are reflected independently of the observation point. The radiation temperature of the ground is generally identical to the remaining thermal background. By transmitting this temperature to the object to be camouflaged, a high contrast reduction can be achieved, with a corresponding gain in camouflage effectiveness. In this case, known LE (Low Emission) camouflaging devices can be used, such as LEP (Low Emission Paint) or LEF (Low Emission Foil).
Known low emission camouflaging devices cannot easily be used for surfaces with a predominantly horizontal orientation, because these surfaces, when observable, always reflect predominantly celestial temperatures close to the zenith. Because such celestial temperatures are very low, and may vary considerably depending on the clouding condition, the reflected heat radiation is extremely dependent on the clouding condition. In many cases, horizontal surfaces which are provided with low emission camouflaging devices will therefore have xe2x80x9ccold spotsxe2x80x9d if, as a result of the reflection of the cold sky, the intrinsic emission is overcompensated. A low emission behavior is desirable only to the extent that a reduction of the thermal radiation is necessary, due to increasing solar heating of the surface.
Similar problems exist in the case of surfaces which are oriented upward (angle to the horizontal line smaller than approximately 65xc2x0), which can also reflect the celestial radiation.
It is therefore an object of the invention to provide a camouflage system for object surfaces which are essentially oriented horizontally or upward.
Another object of the invention is to provide a camouflage system by which effective camouflaging can be achieved without required measuring and regulating devices.
These and other objects and advantages are achieved by the camouflage system according to the invention, in which a material or a layer system used on the surface of the camouflaging device is characterized by a thermal emissivity xcex5 (T) that has a considerable temperature dependence, with a negative gradient (d/dT) (referred to herein as xe2x80x9cthermorefractive materialxe2x80x9d).
As known, the total quantity of heat Q emanating from a body is composed of the intrinsic radiation (product of and the fourth power of the surface temperature TO) and of the reflected ambient radiation (product of 1xe2x88x92xcex5 and the fourth power of the temperature of the reflected-in ambient zone TU, here typically the sky):
Q(T)xcx9cxcex5(TO ),TO4+(1xe2x88x92xcex5(TO)),TU4
(The temperatures above relate to the absolute temperature scale.)
If the body is observed by a thermal imager, this law determines the brightness and the contrast function of the individual picture element, and thus the IR signature of the object.
In the case of normal surfaces with xcex5xe2x86x921, the intrinsic radiation (which increases considerably with temperature) is predominant. According to the invention, a negative temperature coefficient of the thermal emissivity is introduced, and thus the temperature course Q(T) is compensated to the greatest extent possible. If only intrinsic radiation existed, the condition for (T) would have to be:
xcex5(T)xe2x88x92TOxe2x88x924.
However, because the reflection term has to be taken into account, the function (T) may extend with a weaker power. More precise estimates indicate that even a linear reciprocal function
xcex5(T)xe2x88x921/TO
causes a very useful camouflaging effect in practice.
It is important that the thermal emissivity of the overall system decreases markedly within a temperature range which is typically approximately 20 to 40xc2x0 C.; for example, the emissivity may decrease from values xcex5xe2x89xa70.7 to values xcex5xe2x89xa60.5 (in a specific example, from xcex5=0.90 to xcex5=0.5). The lower threshold temperature of the transition range is advantageously equated to the median ambient temperature.
Different mechanisms for achieving a negative temperature coefficient are conceivable in practice. For example, a nonmetalxe2x80x94metal phase transition (MNM transition) can be used. At ambient temperatures, the material is in the nonmetallic or semiconducting condition (IR transparent), and a normal high emission behavior exists when the thermorefractive material is arranged in front of a high emission background. With increasing solar heating, a transition takes place into the metallic condition (IR-reflective) with a resulting lowering of the emissivity. Such a material, which is suitable for the invention, and shows the described MNM transition, is, for example, vanadium oxide (VO2).
Another embodiment of a suitable thermorefractive medium is a composite medium consisting of an IR-transparent matrix, preferably of polyolefine (such as polyethylene) and a dispersed second constituent. The second constituent consists of an alternative organic or polymeric material, also having an IR transparency which is also as good as possible, but with a different temperature course of the refractive indices. For this purpose, liquid, wax-type or semicrystalline hydrocarbons can be used to advantage; however, other substances of low IR absorption in the wavelength range of from 8 to 12 xcexcm are suitable. The material pairing of matrix and dispersion must be coordinated such that the refraction indices of both materials are approximately identical at ambient temperature but deviate increasingly from one another with rising temperature. Such a system exhibits the desired negative temperature effect: At low temperatures, the material is homogeneously IR-transparent andxe2x80x94if the thermorefractive material is arranged in front of a high-emission backgroundxe2x80x94a normal high-emission behavior will exist. At a higher temperature, the amount of scattering will increase, which results in an increased remission, and thus a lowering of the emissivity. In order to take full advantage of the scattering effect, the dispersions should be significantly larger than the infrared wavelength of approximately 10 xcexcm which is relevant to the heat image camouflaging. A suitable size for the dispersions is therefore particularly the range greater than 20 xcexcm.
Because of the temperature-dependent self-regulation of the camouflaging device according to the invention, no additional electronic control system, such as sensors, actuators, triggering electronics and cabling are required. Rather, the emissivity required for an effective camouflaging (and thus the radiation temperatures) will occur automatically. Also, precise site-resolved determination of the surface temperature, which is required by the initially mentioned camouflaging device to adjust the thermal emissivity for each actively controllable IR camouflaging element, is eliminated.
Additional advantages of the invention are:
a highly effective IR camouflaging is achieved for disparate objects;
the camouflaging device according to the invention can be implemented in the form of cost-effective robust elements; and
additional visual camouflaging can be added, in any color.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.